We have developed a nonmyelosblative allogeneic hematopoietic stem cell transplant (HSCT) regimen using low dose total body irradiation (TBI), 200cGy before and immunosuppressive agents, mycophenolate mofetil (MMF)and cyclosporine A (CSP) after grafting in a canine model based on the facts that both host versus graft (HVG) and graft versus host (GVH) reactions are mediated by T cells in the major histocompatible complex (MHC) identical setting. This regimen has now been translated successfully to clinical setting. However, only a minority of patients has human leukocyte antigen (HLA)-identical siblings who could serve as HSCT donors. The present proposal, therefore, seeks to address the development of a nonmyeloablative allotransplant regimen for MHC- haploidentical donors using dog leukocyte antigen (DLA)-haploidentical littermates. In that setting, host natural killer (NK) cells play an important role in HVG reactions, in addition to that played by host T-lymphocytes. We hypothesize that successful HSCT can be achieved by blockades of host T and NK cell immunities without myeloablation. We shall explore three approaches which will either reduce the immune reactivity of host T- cells, host NK cells or both. In specific aim one, we attempt to block host T cell reactions to donor alloantigens with costimulatory blockers, CTLA4lg and/or anti-CD40L monoclonal antibody (mAb). In specific aim two, newly developed anti canine killer mAbs will be tested to block host NK immunities. In specific aim three, a novel immunosuppressive agent, FTY 270 that induced profound peripheral blood lymphopenia involving both T and NK cells will be used to reduce host T and NK cells. The studies will commence using a low dose (450cGy) pretransplant TBI. We shall then attempt to successfully lower the TBI dose needed for successful engraftment and ultimately to establish engraftment solely with the use of non-toxic immunosuppressive agents. Post grafting immunosuppression with MMF1CSP will be an unalterable part of the proposed studies given its demonstrated benefit in preventing GVH disease (GVHD) and reducing HVG reactions. The primary end points of these studies will be stable engraftment that can be easily documented by DNA microsatellite marker analysis and the overall survivals. If successful, the regimen will be translated to humans and provide almost all patients requiring allogeneic HSCT a donor from family members. The applicant is committed to establishing an independent career in academic research as a clinician scientist in the area of hematopoietic stem cell transplantation. The long- term goals are to carry out translational studies that will lead to improvements in the treatment of cancer and other diseases with stem cell transplantation.